The present invention relates to an automatic braking procedure and apparatus, and more particularly, to a method and apparatus for automatic braking of a motor vehicle with an anti-lock system, an automatic braking procedure being carried out on fulfillment of a trigger criterion, a higher pressure than that which corresponds to the position of the brake pedal being produced during the automatic braking procedure, and the automatic braking procedure being ended on fulfillment of an abort criterion.
In an automatic braking procedure described in DE 40 28 290 Cl, the brake pressure produced when the driver actuates the brake pedal at a speed which exceeds a threshold is above the brake pressure which corresponds to the position of the brake pedal. This is intended to reduce the braking distance of the vehicle in the case of emergency braking operations such as those which are carried out by the driver in emergency situations, e.g. to avoid collisions. The known automatic braking procedure is aborted when the pedal return speed exceeds a certain threshold. For example, the maximum brake pressure which can be produced is limited by the anti-lock system (ABS) such that the locking up of wheels is just prevented. Further trigger and abort criteria for an automatic braking procedure are contained, for example, in German Patent Application DE-P. 43 25 940.5.
The conventional automatic braking procedure shortens the braking distance of a vehicle because a certain brake pressure to be achieved is produced at least more quickly than is the case with a corresponding actuation of the brake pedal by the driver. Furthermore, higher brake pressures are generally produced since drivers frequently do not brake sufficiently hard. The braking operation which takes place in this way utilizes the information on the adhesion coefficient provided by the ABS installed in the vehicle only insofar as the ABS prevents locking up of the wheels during the automatic braking procedure by virtue of the fact that the reaching of the locking limits at one wheel prevents a further rise in the brake pressure.
It is an object of the invention to markedly improve the known automatic braking procedure such that the braking distance of the vehicle is further reduced by the automatic braking procedure.
This object has been achieved by the fact that the brake pressure produced during the performance of the automatic braking procedure is increased at each axle to an extent such that the anti-lock system detects the reaching of the locking limits of at least one wheel of the respective axle. A reduction in the braking distance as compared with the known automatic braking procedure is accordingly achieved by the fact that the brake pressure is not only increased to such an extent that one wheel of the vehicle reaches the locking limit but to such an extent that the brake pressure of the individual axles is increased until one wheel of this axle reaches the locking limit. The maximum transmission of force between the wheel and the roadway is thus utilized at each axle. In the case of passenger vehicles, it depends above all on the configuration of the brake system of the vehicle and, in the case of lorries, above all on the cargo and on the load distribution of the cargo at which of the axles of the vehicle the locking limit is reached first.
In the present invention, the locking limit of a wheel is to be regarded as the torque at which the slip between the wheel and the road becomes so great that a further rise in brake pressure triggers regulation of the braking force at this wheel or at the axle of the wheel by the ABS. According to an embodiment of the present invention, the brake pressure is produced by increasing the brake pressure in the entire brake circuit until the locking limit of at least one wheel of the axle which locks first is reached. The brake pressure at the axles which have not yet locked is then increased by feeding brake pressure medium into the corresponding brake cylinders from a pressure source. At driven axles it is advantageously possible to make use of the devices for traction control.
The method according to the present invention can, of course be employed on vehicles with any number of vehicle axles. It is also possible in this context to group axles bearing essentially the same load such as tandem or twin axles. These axles then have a common pressure source, by way of which the same common brake pressure is produced in the brake cylinders of the wheels. A common pressure source can supply a number of axles with braking medium if at least one control valve is used to regulate the brake pressure for each axle.